Saiunkoku Monogatari

Saiunkoku Monogatari - anime series based on japanese novels written by Sai Yukino and illustrated by Kairi Yura. In brief: Kou Shuurei Kou, nice and pretty girl, dreams about passing through the imperial examinations and taking a post in government. If you're interested in the anime, a Wikipedia article contains much more information.

Kou Kijin

Kou Kijin (Kijin Kou) is not deeply involved in the main plot. One day Shuurei disguises herself as a boy and works in the Department of Finances, under Kou Kijin's command. This mysterious, eccentric person always wears a mask. People used to think that it's because he's ugly, but actually he's so beautiful that everyone falls in love with him, even men. People almost go mad, they have no peace and betray their families and wives.

Several screencaps:

The title

When Kou Kijin took the imperial exams, all other candidates couldn't stop looking at him and were unable to complete their tasks. That is why almost all candidates (except Kou Kijin himself, of course) failed to pass. This incident caused that exam year to be referred to as The Nightmare Exam.